Wireless communication technologies, such as those used for NFC or ISO/IEC 14443 devices, communicate with each other via magnetic field induction in close distance. Each device features an antenna. The primary device (“reader” or “initiator”) generates the magnetic field which can be used to power secondary devices like passive transponders. Modulation schemes applied to the magnetic fields are used for communication purpose between the devices.
The primary device uses a transmitter to generate the emitted radio frequency (RF) field. A matching circuitry is used to transform and adapt the antenna impedance to the emitting device's transmitter. A low ohmic matching impedance is generally used for increased power transmission.
One limiting factor of the operating volume size is the emitted RF power of the primary device. Applications like contact-less payment systems require a specific operating distance. As the primary device's antenna size is often a constraint to the product, high output power transmitters are required.
The primary and secondary devices form a coupled wireless resonant circuit. If the coupling between the components is increased, the primary resonant circuit will be loaded and detuned. This results in a different load impedance seen by the primary device's transmitter, possibly causing an increased driver current and increased RF field emission.
Typically, the primary device use a periodic startup to check if any secondary device (e.g., a NFC card) is in the proximity. The checking may include measuring transmitter current, phase of the career wave, receiver voltage, etc. because these parameters are likely to be changed due to changes in antenna impedance when the secondary device is in the operational proximity. However, a full startup of the primary device frequently causes a higher energy usages. In addition, these methods do not offer high sensitivity, especially with modern small antennas.